I swapped a ramcharger tank into a '76 Club Cab I had back in the '90's.

If you find a donor ramcharger and it has a metal tank, its the smaller tank. All 35 gallon tanks were plastic.

Either tank uses the same crossmembers, just the straps are different.

And I'm not 100% sure, but, I think the smaller tank was used predominantly on two wheel drive RC's (mid '70's, not the later RC's).

If you go with the RC tank, you need to swap in the crossmembers from the RC. The tank will dictate where they need to go. The hardest part of swapping the crossmembers is getting the factory rivets out.

If you decide to go Mustang, disregard what I just said....